A clear warning to industry not to invest money and energy in the proposed third runway at Heathrow has been issued by Tory leader David Cameron as he reaffirmed his party’s commitment to scrap the project if it wins power.

Some observers have suggested that the green light given by Transport Secretary Geoff Hoon last week for expansion represents a guarantee that it will go ahead, as the scheme will be too far advanced by the time of the next election for an incoming Tory administration to halt it.

But Mr Cameron flatly rejected this assumption, stating: “The third runway is just not going to happen.”

In an interview with The Guardian, he insisted he did not believe a Tory Government would be required to pay compensation if it scrapped the scheme.

With Mr Hoon’s announcement triggering what is expected to be a protracted planning process, little financial outlay will have been made by Heathrow’s owner BAA by the election, he suggested.

And he left businesses in no doubt that any investments made on the assumption that the third runway will be built will be put at risk if the Tories win power.

“What business needs to recognise is that the third runway is just not going to happen,” said Mr Cameron.

“There is such a coalition of forces against it. There’s such an environmental case against.”

Mr Hoon’s announcement was greeted with horror by environmentalists, local residents and many Labour backbenchers, who have opposed the scheme on the grounds it will worsen air quality and noise levels around Heathrow and set back Britain’s efforts to cut emissions of climate change gases.

Labour MP John McDonnell, whose Hayes and Harlington constituency includes the airport, was suspended from the Commons for five days after picking up the mace in protest at Mr Hoon’s decision.

Campaigners were today vowing to conduct a campaign of protest and direct action to block the 7,200ft runway.

But airlines, big business, Heathrow operator BAA and some unions all welcomed the announcement, which paves the way for the construction of the runway sometime between 2015 and 2020.